This invention deals generally with power supplies, and more specifically with non-dissipative switching transistor snubbers.
A switching transistor snubber is circuitry added to a switchmode power supply which reshapes the transistor load line to reduce stresses on the switch transistor, and can also reduce electromagnetic interference (EMI) caused by the switchmode power supply. The switch transistor in a switchmode power supply is turning off into an inductive load which causes a transistor load line as shown in FIG. 1 for no snubber. If the load line is not reshaped by a snubber, circuit peak collector voltages are coincident with peak collector currents as shown in FIG. 2, resulting in maximum power dissipated in the switch transistor.
A dissipative resistor-capacitor (RC) snubber circuit in a single-ended flyback converter is shown in FIG. 3. While transistor Q.sub.1 is on, capacitor C.sub.1 is charged to a value of E.sub.in through resistor R.sub.1. When transistor Q.sub.1 is turned off, the current from transformer T.sub.1, which would pass through transistor Q.sub.1 without a snubber, is diverted to capacitor C.sub.1 and resistor R.sub.1. Power typically dissipated by rsistor R.sub.1 is: EQU P=(f) (C.sub.1) (2E.sub.in).sup.2.
An offline converter using: C.sub.1 =2000 pF, f=20 kHz, and E.sub.in (max)=360 V, would find P=20.7 watts.
A dissipative diode-resistor-capacitor (DRC) snubber circuit for a single-ended flyback converter is shown in FIG. 4. The function of the ciruit is similar to the RC snubber circuit. However, the power consumption, although still high, is approximately halved by shunting resistor R.sub.1 with a diode CR.sub.1.
A nondissipative diode-inductor-capacitor (DLC) snubber circuit disclosed in a paper titled "DESIGNING NON-DISSIPATIVE CURRENT SNUBBERS FOR SWITCHED MODE CONVERTERS" by Eugene C. Whitcomb at the Proceedings of the Sixth National Solid-State Power Conversion Conference in May 1979 is shown in FIG. 5. This snubber would probably work in a non-dissipative manner for 115 V AC power, but has drawbacks when operating with 230 V AC power. The voltage at the node between resistors R.sub.1 and R.sub.2 tends to be pulled down by the current drawn through diode CR.sub.2. Consequently, capacitor C.sub.1 would have to be a high voltage capacitor, rated at approximately 300 Volts or more. This part is physically larger than most standard capacitors, and is less common and therefore more expensive. Common objectives in designing power supplies for international markets include efficiency of power and physical space and adaptability to international power standards (115/230 V).